For the past forty years, inorganic silicon and gallium arsenide semiconductors, silicon dioxide insulators, and metals such as aluminium and copper have been the backbone of the semiconductor industry. However, there has been a growing research effort in "organic electronic" to improve the semiconducting, conducting and light emitting properties of organics (polymers, oligomers) and hybrids (organic-inorganic composites) through the novel synthesis and self-assembly techniques. This rich area of research has been benefited from studies on rational design based on crystal engineering, including supramolecular chemistry, and the interest to investigate the structures and properties of organic solids states. This research work was carried out to understand the intermolecular interactions and molecular recognition of solid state structures that might exhibit interesting electrical, magnetic and optical properties. For this purpose, molecules possessing pyridyl groups at the terminal positions have attracted much attention, and their derivatives have been reported by several groups since these molecules can afford supramolecular wires by coordination with metals and/or hydrogen bonding involving the nitrogen atoms of the pyridyl groups. In this context, linear π-conjugated molecules have now been synthesized by inserting benzothiadiazole as spacer units between the dipyridyl backbone. The introducing of 1,2,5-thiadiazole rings is of interest due to their extended m-conjugation and polarized heteroatoms which are expected to afford well-ordered crystal structures leading to intermolecular interactions such as heteroatom contacts or π...π interactions. The compounds containing thiadiazole rings are well known as strong fluorescent materials. Highly fluorescent π-conjugated molecules are of interest from the application purposes such as EL (electrioluminescence) devices and single molecular detection. Moreover, this heterocycle is electron-withdrawing and the compounds bearing this ring arc possible candidates for electron carriers. The electron accepting conjugated molecules are also of interest from view points of the NDR (negative differential resistance) behaviors since recent studies have shown that oligo (phenyleneethylene)s containing an electron-withdrawing nitro group can be used as active redox centers responsible for the NDR behavior in semiconductor devices.
In this research work, the author designed some new dipyridyl compounds and their viologen analogues containing benzothiadiazole spacer units as shown in general scheme l and investigated their electronic properties, crystal structures, and established the relationship between the structures and fluorescence properties. The compounds of 4,7-di(n-pyridyl)-1,2,3-benzothiadiazole (n = 2, 3, and 4) (1a-c ) show high quantum yields of fluorescence and high electron affinities. They also show solid-state fluorescence due to weak intermolecular interactions caused by their large torsion angle between the benzothiadiazole and pyridyl rings. There is no significant effect of the nitrogen positions of the pyridyl rings on the quantum yield of fluorescence. The crystal structures of the compounds 1a-c have the similar space group with overlapping the stacked molecules in a head-to-tail fashion. The reduction potentials of their viologen analogues 2-3 are higher than their corresponding neutral compounds. The quantum yields of fluorescence of these compounds are low due to decrease of the torsion angles.
The compounds 4,7-bis (n-pyridylethynyl)-1,2,3-benzothiadiazole (n = 2, 3, and 4) (4a-c) also show high electron affinities and high quantum yields of fluorescence. The absolute quantum yield of 4c is 0.87± 0.05. The solids-state fluorescence of these compounds is lower compared to the compounds 1a-c. This can be explained by the planarity of the molecular structures. X-ray structures analysis revealed that the torsion angles between the benzothiadiazole and pyridine ring are smaller than those of the compounds 1a-c. So the intermolecular interactions becomes stronger and the solid-state fluorescence decreases. The molecules are stacked to afford unusual two-dimensional columns where the distance between the molecular planes is 3.79 Å. The columns run in two directions with 45°. No short heteroatom contacts such as S...N are observed. The crystal structures of 4a-c are similar although the space groups are different. The noncentrosymmetric space group is interesting from the standpoint of nonlinear optical properties. The molecular structures of their methylated compounds are more planar than the corresponding neutral compounds. Their crystal structures include π...π staking of long molecules. A number of intermolecular O-H...O C-H...O and C-H...F interactions are also found in the methylated structures.
The absorption maxima of bis (benzothiadiazole) compounds (8-11) are observed at longer wavelengths than those of mono(benzothiadiazole) compounds due to longer conjugation in these compounds. The quantum yields are a little decreased compared to the mono(benzothiadiazole) compounds. There is no significant effect of the nitrogen positions on the absorption and emission spectra on these compounds. The reduction potentials of these compounds show stepwise one-electron reduction waves due to the presence of two benzothiadiazole ring, and also higher than those of mono benzothiadiazole derivatives. This is attributed to the electron withdrawing property of the thiadiazole ring. X-ray structure analysis reveals that the bis(benzothidiazole) derivatives are nonplanar structures due to the repulsion between the pyridine and benzothidiazole ring. There are short S-N contacts observed, leading to the molecular tape structures. It is noteworthy that the dihedral angles of their methylated compounds are drastically decreased, and the dihedral angle between the pyridine and benzothidiazole ring is absolutely zero degree in the methylated compound 11c.
Finally, the author used dipyridyl compounds containing π-conjugated groups as proton acceptor as shown in general Scheme 2, and investigated their complexation with chloranilic(CLA) and squaric (SQA) acid by hydrogen-bonding interactions which are very important in the field of crystal engineering as well as in supramolecular chemistry. The introduction of a π-conjugated spacer to dipyridyl compounds have been considered to increase intermolecular π-π interactions and decrease Coulombs repulsion in the dication states. The crystal structures are strongly dependent on the spacer group. Although the details of relationship between the crystal structures and spacer group are still ambiguous, it should be noted that the structure of the complexes of the twisted molecules are completely different from those of the planar molecules. The author believed that more elaborated crystal engineering would be possible by changing the spacer group and/or substituents of anilic acid.